Very intermittent brief loss of shower pressure

Before getting a local plumber involved, I'd very much appreciate some thoughts on potential causes of my issue. Thanks in advance.

1. When taking a long shower, there is an almost split second sudden drop in pressure (as if someone turned OFF the water supply then immediately turned back on, but even more brief than the time this would take). This is similar to the effect of flushing the toilet in the old days and having a split second of low pressure (but I don't get, for example, all HOT or all COLD, it is ALL the water going off like holding your hand in front of the spray for a second to simply "stop" the spray.

2. More details.

o This happens only once every 7 to 10 showers, not every time. When it happens, it doesn't feel like all cold or all hot remains, it is more like all water stops, but so briefly that the shower head is never completely off (that is, it kicks back in almost immediately). Otherwise all pressure in shower and rest of the house is very strong and consistent.

o If I try to create the problem by flushing toilet, turning on other taps, etc. I can't. That is, I do NOT lose any shower pressure from flushing toilets, running other taps.

o This is an old house, but all the plumbing was done from scratch about 2 years ago, including line to the street meter, new hot water heater (electric), gut renovation of bathroom/shower, new feed lines, fixtures, etc.

o Problem only appeared in the last month or so, but it is so intermittent, that it could have always been around and simply didn't happen while showering.

o I haven't noticed any loss of pressure when using kitchen or sink in bathroom, but then again, I'm never running these taps for as long as I run the shower.

I would check for a toilet that has a leaking flapper. This could be a toilet tank refilling on it's own.

And if this home has been repiped, I would love to see the sizing you used. If the home was piped properly, you wouldn't be having this pressure loss.

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definitely not toilet (checked for this and no running whatsoever ....and by the way we use your suggested toto toilets!). Not sure about piping size. If this was under sized wouldn't we be having lots of pressure drops, not just a split second once every 7 to 10 days?

Other things that come on at odd times are"
Washing machine
Dishwasher
Icemaker

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definitely not dishwasher (don't have one, odd I know, but don't like them) or washing machine (my first thought was washing machine too). I suppose it could be icemaker, but rarely use ice, and stays full for weeks on end so rarely runs.

I'm in Florida. No furnace (well, an electric heat pump for AC/heat) and no humidifier. I do have an external irrigation system, but it is not on during the brief pressure loss and have noticed no leaks (and if a leak, would be consistent I suppose rather than very intermittent)

When it happens, it only happens toward the end of a long shower (not super long, maybe 10 minutes at the very most). I wonder if something could be happening with the Electric Hot Water Heater such that some sort of refill is taking place (but tank is a large one, can't recall, but it is at least a 50 gallon model).

Some crud caught in the supply line, too big to flush out, but depending on the flow, might work its way where it could shut things off. If shaped just right, might do that. Depending on the slope of the line, might fall back a long ways when the flow stops, then work its way back to where it could cause problems.

Some crud caught in the supply line, too big to flush out, but depending on the flow, might work its way where it could shut things off. If shaped just right, might do that. Depending on the slope of the line, might fall back a long ways when the flow stops, then work its way back to where it could cause problems.

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could be. I've carefully determined that the ideas about other things randomly using water (washer, ice maker, etc.) are not a possibility. And note that even if I intentionally turn on other water taps or flush toilet, this does NOT cause any drop in shower pressure. Again, 99.9% of the time, all works perfectly.

Is this something that is caused by my shower mixer or is this not very likely? I'm going to try to let the sink run for a long time to see if I get a similar problem there (as a test of whether the problem is the shower valve).

Is there a pressure regulator on the feed from the street? If so, it may be getting sticky. If not, I suspect the pressure balance spool in the Hansgrohe. Still, if it is the spool, something must be affecting the pressure to set it off.

I would also suspect an obstruction in the piping. In my opinion it is most probable that it is somewhere in the piping near the main service line. The reason I suspect this is because both hot and cold are affected. Hot and cold are both dependant upon the main service. It could be that your shower is a high volume user compaired to a faucet or toilet refilling. Especially if the flow restrictors have been removed from your shower. The valve is capable of high flow being rated at over 13 gallons per minute. It may take lot's of flow to trigger the obstruction to close. Try running all of your faucets and the shower at once. flush toilets too. If you can recreate the error while running lots of water, I think you have a problem somewhere in the main water service. If you can't get it to cause the problem anywhere but the shower, I believe you may have a delaminating shower hose. The Techniflex shower hose is many layered. If you have a kink internally it could be that high flow plus the heat of a long shower will allow it to collapse internally. If this is the case, or before you start taking your water service apart, call Hansgrohe at 800-334-0455 and ask for a replacement hose. They stand behind their products 100% and will send it to you at no charge under the warranty. I should know. I am the manager of Technical Service for them. There is no possibility that this is an error with the shower valve in my opinion.

Phil, many thanks for your response. I'll try your suggestions and your explanations as to cause make lots of sense. I'm pleased to hear that it is very unlikely to be the fairly new shower valve (as replacing this may cause issues with the very expensive new tile job!)